Processor utilization grows over months of use?

I just looked at the processor utilization of my pfsense box and was shocked to see the "nice" utilization had grown to be about 40-50% of my CPU. I have a Via-based pfSense box that does modest OpenVPN duty. The RRD graph shows it went from about 1-3% utilization to the 50% level over a period of months. I rebooted the box and it fell back to the 1-3% level.

What would have been interesting was to see what process was utilizing the CPU, via top or similar.

Actually, I looked for that. When I logged in via SSH, tho, I did not see the utilization shown on the graph. I did see an instance of sh which was "niced" and it was showing something like 10% utilization, but that was probably my SSH session.

I should mention that at no time did I suspect anything was wrong because my pfSense box was routing with no perceived degradation. I only became nervous when I saw the RRD graph and rebooted the box. It's now averaging 0.13% nice, 1% system, 0.25% user, and 0.62% interrupt.